1. Field of the Invention
The present invention relates to controlling the ignition rate of a solid propellant rocket grain and, more particularly, this invention relates to the vapor deposition of a thin polymer film on the surface of a rocket grain from a vaporized monomer.
2. Description of the Prior Art
The maximum performance of future missiles is limited by booster rocket and overall system constraints such as the ignition mass flow rise rate. This rate is typically controlled by an inhibitor on the surface of a solid propellant rocket grain.
For the purposes of the present invention, such a grain is defined as a relatively large mass of solid propellant material, which is shaped into a suitable geometric pattern and is used by itself or with only a few other corresponding and relatively large masses, rather than as one particle of a very large number of propellant particles which, for use, may be poured or packed into any suitable container or which are for subsequent incorporation in a binder. A grain of the present invention may be tubular or have a more complex centrally perforated shape or may have the configurations known in the art as cruciform, star, and multitubular.
U.S. Pat. No. 3,523,839, which issued Aug. 11, 1970 to Shechter et al and which is hereby incorporated by reference, describes the application of coatings of paraxylylene polymers to particulate propellant components by condensation thereon of monomers produced by pyrolytic cleavage of the corresponding dimer. However, this patent only discloses the application of such polymers to propellant components which are in particulate form and which are kept in continuous motion in a coating chamber during subjection of the particle exteriors to the monomer.
The general state of technology relating to the inhibition of ignition of a solid propellant rocket grain is either to paint some material on the surface of the grain, or to coat a mandrel with an inhibitor and cast the propellant onto it. The most common method is to paint a liner material on the grain surface. These methods are not uniformly controllable and are labor intensive. The liner material requires a separate cure and it is also very difficult to inspect the liner coating to ascertain whether it is uniformly applied and in the correct thickness. The liner coating formulations contain compounds that can react with and/or migrate into or out of the propellant surface, and thus cause degradation of underlying propellant over time.
It is particularly difficult and expensive to apply a uniformly thick liner coating to shaped or large rocket propellant grains so that the design of rocket motors has been constrained to special configurations to satisfy the limits imposed on the duration of the ignition transient.